The metal backed cathode ray tube is manufactured by depositing a metal layer such as an aluminum layer on the back of the luminescent layer in order to improve the luminance, to reinforce the potential, and to prevent the burning of the luminescent layer. An example of such a metal backed color cathode ray tube is disclosed in Japanese Patent Publication No. Sho 56-25736, the constitution of which is illustrated in FIG. 1. According to this technique, graphite is spread in the form of stripes on the inner surface of the face plate F of the panel P, thereby forming a black matrix B. This metal backed color cathode ray tube comprises: the above mentioned black matrix B; a luminescent layer L formed by alternately depositing R,G,B luminescent materials containing an organic ingredient such as PVA as the base through the use of a photo etching method; a filming layer M composed of organic ingredients such as acryl emulsion for separating the luminescent layer L from a deposited metal layer A to be described later; and a deposited metal layer A made of aluminum layer and the like and deposited through the use of an electric deposition method so as for the layer A to serve as a metal back.
In the finishing process, such a panel is sealingly joined with a funnel (not shown), thereby forming a bulb. However, if impurities adhered to the seal edge E during the joining, cracks will be produced, and therefore, there will be the risk that the cathode ray tube might implode. Therefore, the portions of the layers deposited on unnecessary areas during the above described spreading processes are removed by proper methods. That is, the graphite forming the black matrix B is cut away by means of a chemical such as ammonium acid fluoride (NH.sub.4 FHF), and the superfluous portions of the luminescent layer L and the filming layer M are washed off by means of a wiper of high pressure water, while the deposited metal layer A is deposited in such a manner that a proper shielding plate is installed on the deposition dolly so as for only the required areas to be deposited.
However, a stud pin T for installing a shadow mask (not shown) protrudes from a skirt S of the panel P, and therefore, it is difficult to carry out a complete washing by means or wiper with high pressure water, with the result that serious problems are generated during the baking process due to the residue of organic materials of the luminescent layer and the filming layer.
A baking is carried out to burn off (remove) the organic materials such as PVA and acryl emulsion by heating the panel to a high temperature, so that the electron beams emitted from the electron gun should not lose their energies due to the organic materials before they reach the luminescent layer. However, if a deposited metal layer A is formed upon the surface of the residue organic materials O adhered on the skirt S, then the deposited metal layer will swell up to float above the residue organic material during the baking process due to the combustion gas of the organic materials, and this floated-up metal layer A will be detached away after the manufacturing of the cathode ray tube to block up the holes of the shadow mask, this having been one of the main factors for the hole-blocking defects. Therefore, in order to remove the detached materials, a salvaging process had to be carried out, with the adverse result that much labor and materials had to be squandered. According to the investigations carried out by the present inventor, the defective product rate due to the detachment of the deposited metal layer A such as aluminum layer occupies 20.about.30% of the total hole-blocking defects.